Light emitting element and light emitting device using the same

ABSTRACT

An object of the present invention is to provide a light emitting element having slight increase in driving voltage with accumulation of light emitting time. Another object of the invention is to provide a light emitting element having slight increase in resistance value with increase in film thickness. A light emitting element of the invention includes a first layer for generating holes, a second layer for generating electrons and a third layer comprising a light emitting substance between first and second electrodes. The first and third layers are in contact with the first and second electrodes, respectively. The second and third layers are connected to each other so as to inject electrons generated in the second layer into the third layer when applying the voltage to the light emitting element such that a potential of the second electrode is higher than that of the first electrode.

TECHNICAL FIELD

The present invention relates to a light emitting element comprising alayer that includes a light emitting substance between a pair ofelectrodes, and in particular, relates to a structure of a lightemitting element.

BACKGROUND ART

In recent year, many light emitting elements used for display devicesand the like have a structure in which a layer that includes a lightemitting substance is sandwiched between a pair of electrodes. Such alight emitting element emits light when an excited electron, which isformed by a recombination of an electron injected from one electrode anda hole injected from the other electrode, returns to a ground state.

Many of these light emitting elements have a problem in that the drivingvoltage is increased with the accumulation of light emitting time.

In order to solve this problem, for example, the patent document 1discloses an organic EL element using a compound with a certainstructure, wherein the increase in driving voltage, and the like aresuppressed in driving the organic EL element.

-   [Patent Document 1]: International Patent Publication No. WO98/30071

DISCLOSURE OF INVENTION

It is an object of the present invention to provide a light emittingelement having a slight increase in driving voltage with theaccumulation of light emitting time. It is another object of theinvention to provide a light emitting element having a slight increasein resistance value with the increase in film thickness.

In an aspect of the invention, a light emitting element includes a firstlayer, a second layer and a third layer between a first electrode and asecond electrode that are provided to face each other. The first, secondand third layers are laminated to one another while sandwiching thesecond layer between the first and third layers. The first layer is incontact with the first electrode and the third layer is in contact withthe second electrode. The first layer generates holes while the secondlayer generates electrons. The third layer includes a light emittingsubstance. The second layer and the third layer are contacted to eachother so as to inject electrons generated in the second layer into thethird layer when applying the voltage to the light emitting element suchthat a potential of the second electrode is higher than that of thefirst electrode. By contacting the second layer to the third layer, thelight emitting element emits light when being applied with a voltagesuch that a potential of the second electrode is higher than that of thefirst electrode.

In another aspect of the invention, a light emitting element includes afirst layer, a second layer and a third layer between a first electrodeand a second electrode that are provided to face each other. The first,second and third layers are laminated to one another while sandwichingthe second layer between the first and third layers. The first layer isin contact with the first electrode and the third layer is in contactwith the second electrode. The first layer includes a substance of whicha hole transporting property is stronger than an electron transportingproperty, and a substance having an electron accepting property withrespect to the substance of which the hole transporting property isstronger than the electron transporting property. The second layerincludes a substance of which an electron transporting property isstronger than a hole transporting property, and a substance having anelectron donating property with respect to the substance of which theelectron transporting property is stronger than the hole transportingproperty. Also, the third layer includes a light emitting substance. Thesecond and third layers are contacted to each other so as to injectelectrons generated in the second layer into the third layer whenapplying the voltage to the light emitting element such that a potentialof the second electrode is higher than that of the first electrode. Bycontacting the second layer to the third layer, the light emittingelement emits light when being applied with the voltage such that thepotential of the second electrode is higher than that of the firstelectrode.

In another aspect of the invention, a light emitting element includes afirst layer, a second layer and a third layer between a first electrodeand a second electrode that are provided to face each other. The first,second and third layers are laminated to one another while sandwichingthe second layer between the first and third layers. The first layer isin contact with the first electrode and the third layer is in contactwith the second electrode. The first layer includes a p-typesemiconductor and the second layer includes an n-type semiconductor. Thethird layer includes a light emitting substance. The second and thirdlayers are contacted to each other so as to inject electrons generatedin the second layer into the third layer when applying the voltage tothe light emitting element such that a potential of the second electrodeis higher than that of the first electrode. By contacting the secondlayer to the third layer, the light emitting element emits light whenbeing applied with the voltage such that the potential of the secondelectrode is higher than that of the first electrode.

In the above-described light emitting element of the invention, thelayer containing the light emitting substance may have a single layer ormultiple layers. When the layer containing the light emitting substancehas multiple layers, the light emitting substance may be included atleast in one layer of the multiple layers.

In another aspect of the invention, a light emitting element includes afirst layer, a second layer and a third layer between a first electrodeand a second electrode that are provided to face each other. The first,second and third layers are laminated to one another while sandwichingthe second layer between the first and third layers. The first layerincludes a substance of which a hole transporting property is strongerthan an electron transporting property, and a substance having anelectron accepting property with respect to the substance of which thehole transporting property is stronger than the electron transportingproperty. The second layer includes a substance of which an electrontransporting property is stronger than a hole transporting property, anda substance having an electron donating property with respect to thesubstance of which the electron transporting property is stronger thanthe hole transporting property. The third layer has x pieces of layers(x is a given positive integer) including a light emitting layer. Onelayer included in the third layer is in contact with the second layerand the x^(th) layer thereof is in contact with the second electrode.The first electrode includes a conductive material having highreflectance. There are y pieces of layers (y<x wherein y is a positiveinteger) between the light emitting layer of the third layer and thesecond layer. The second layer and the one layer of the third layercontacting to the second layer are in contact with each other so as toinject electrons generated in the second layer into the one layer of thethird layer when applying the voltage to the light emitting element suchthat a potential of the second electrode is higher than that of thefirst electrode. By contacting the second layer to the one layer of thethird layer, the light emitting element emits light when being appliedwith the voltage such that the potential of the second electrode ishigher than that of the first electrode. Also, the thicknesses of thefirst and second layers are adjusted to satisfy the followingexpressions 1, 2 and 3:

$\begin{matrix}{{{n_{i}d_{i}} + {n_{ii}d_{ii}} + {\sum\limits_{k = 1}^{y}\;{n_{k}d_{k}}} + {n_{j}d_{j}}} = \frac{\left( {{2\; m} - 1} \right)\lambda}{4}} & 1 \\{0 \leqq d_{j} \leqq d_{emi}} & 2 \\{d_{i} \geqq d_{ii}} & 3\end{matrix}$

In the expressions 1, 2 and 3, n_(i) indicates the refractive index ofthe first layer; d_(i), the thickness of the first layer; n_(ii), therefractive index of the second layer; d_(ii), the thickness of thesecond layer; n_(k), the refractive index of the k^(th) layer of thelayers sandwiched between the light emitting layer and the second layer;d_(k), the thickness of the k^(th) layer of the layers sandwichedbetween the light emitting layer and the second layer; n_(j), therefractive index of the light emitting layer; d_(j), a distance betweena first-electrode-side surface of the light emitting layer and a lightemitting region; λ, a wavelength of light emission from the lightemitting element; m, a given positive integer; and d_(emi), thethickness of the light emitting layer.

According to the present invention, a highly reliable light emittingelement having slight increase in driving voltage with the accumulationof light emitting time can be obtained.

In addition, a light emitting element having slight increase inresistance value that is dependent on the thickness of a layergenerating holes can be obtained according to the invention. As aresult, a light emitting element in which a distance between electrodescan be changed easily can be obtained. Also, by increasing the distancebetween the electrodes, the short-circuiting between the electrodes canbe prevented. Additionally, by controlling the distance between theelectrodes, an optical distance can be easily controlled such that thelight extraction efficiency can be increased to a maximal value. Inaddition, by controlling the distance between the electrodes, an opticaldistance can be controlled easily so that the variation in emissionspectrum depending on an angle of seeing a light emitting surface isreduced.

Furthermore, by applying a light emitting element obtained according tothe present invention to a light emitting device, a highly reliablelight emitting device that can withstand long-time use can be obtained.Moreover, by applying the light emitting element obtained according tothe invention to a light emitting device having a display function, itis possible to obtain a light emitting device capable of displayinghigh-definition images with slight variation in the emission spectrumthat depends on an angle of seeing a light emitting surface, whereinlight can be emitted to the outside efficiently.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a view showing a laminated structure of a light emittingelement according to the present invention;

FIG. 2 is a view showing a laminated structure of a light emittingelement according to the present invention;

FIG. 3 is a view explaining a light emitting device according to theinvention;

FIG. 4 is a diagram explaining a circuit included in a light emittingdevice according to the invention;

FIG. 5 is a top view of a light emitting device according to theinvention;

FIG. 6 is a diagram explaining a frame operation of a light emittingdevice according to the invention;

FIGS. 7A to 7C are cross sectional views of light emitting devicesaccording to the invention;

FIGS. 8A to 8C are diagrams showing electronic appliances according tothe invention;

FIG. 9 is a graph showing the voltage-luminance characteristics of alight emitting element according to the invention;

FIG. 10 is a graph showing the current density-luminance characteristicsof a light emitting element according to the invention;

FIG. 11 is a graph showing the voltage-current characteristics of alight emitting element according to the invention;

FIG. 12 is a graph showing results obtained by measuring the change involtage with time of a light emitting element according to theinvention;

FIG. 13 is a graph showing results obtained by measuring the change inluminance with time of a light emitting element according to theinvention;

FIG. 14 is a view showing a laminated structure of a light emittingelement according to the invention;

FIG. 15 is a view showing a laminated structure of a light emittingelement according to the invention;

FIG. 16 is a graph showing the voltage-luminance characteristics of alight emitting element according to the invention and a light emittingelement according to the comparative example;

FIG. 17 is a graph showing the voltage-current characteristics of alight emitting element according to the invention and a light emittingelement according to the comparative example;

FIG. 18 is a view showing a laminated structure of a light emittingelement according to the invention;

FIG. 19 is a graph showing the voltage-luminance characteristics of alight emitting element according to the invention;

FIG. 20 is a graph showing the voltage-current characteristics of alight emitting element according to the invention;

FIG. 21 is a graph showing the luminance-current efficiencycharacteristics of a light emitting element according to the invention;

FIG. 22 is a graph showing results obtained by measuring the change incurrent efficiency (cd/A) with respect to the distance (nm) of a layer775 to a first electrode 778;

FIGS. 23A to 23C are graphs showing results obtained by measuring thechange in shape of light emission spectrum depending on an angle ofseeing a light emitting surface;

FIG. 24 is a view showing a laminated structure of a light emittingelement according to the invention;

FIG. 25 is a graph showing the voltage-luminance characteristics of alight emitting element according to the invention; and

FIG. 26 is a graph showing the voltage-luminance characteristics of alight emitting element according to the invention.

BEST MODE FOR CARRYING OUT THE INVENTION

The embodiment modes according to the present invention will hereinafterbe described referring to the accompanying drawings. It is easilyunderstood by those who skilled in the art that the embodiment modes anddetails herein disclosed can be modified in various ways withoutdeparting from the purpose and the scope of the invention. The presentinvention should not be interpreted as being limited to the descriptionof the embodiment modes to be given below.

Embodiment Mode 1

One embodiment mode of the present invention will be described withreference to a cross sectional view of a light emitting element as shownin FIG. 1.

The light emitting element includes a first layer 111, a second layer112 and a third layer 113 between a first electrode 101 and a secondelectrode 102. The first, second and third layers are laminated to oneanother. The first layer 111 is in contact with the first electrode 101and the third layer 113 is in contact with the second electrode 102.

The light emitting element of the present embodiment mode is operated asfollows. When the voltage is applied to the light emitting element suchthat a potential of the second electrode 102 is higher than that of thefirst electrode 101, holes are injected into the first electrode 101from the first layer 111 while electrons are injected to the third layer113 from the second layer 112. Also, holes are injected to the thirdlayer 113 from the second electrode 102. The holes injected from thesecond electrode 102 and the electrons injected from the second layer112 are recombined in the third layer 113 so that a light emittingsubstance is excited. The light emitting substance emits light whenreturning to a ground state from the excited state.

Thereinafter, the various layers, electrodes and the like will bedescribed in more detail below.

The first layer 111 generates holes. As the first layer 111, forexample, a layer containing a substance with a hole transportingproperty and a substance having an electron accepting property withrespect to the substance with the hole transporting property can begiven. The substance with the hole transporting property indicates asubstance of which a transporting property is stronger than an electrontransporting property. The substance with the hole transporting propertyis not particularly limited. For example, an aromatic amine compoundsuch as 4,4′-bis(N-[1-naphthyl]-N-phenylamino) biphenyl (abbreviation:NPB), 4,4′-bis(N-[3-methylphenyl]-N-phenylamino)biphenyl (abbreviation:TPD), 4,4′,4″-tris(N,N-diphenylamino)triphenylamine (abbreviation:TDATA), 4,4′,4″-tris(N-[3-methylphenyl]-N-phenylamino)triphenylamine(abbreviation: MTDATA), and4,4′-bis(N-(4-[N,N-di-m-tolylamino]phenyl)-N-phenylamino)biphenyl(abbreviation: DNTPD); a phthalocyanine compound such as phthalocyanine(abbreviation: H₂Pc), copper phthalocyanine (abbreviation: CuPc) andvanadyl phthalocyanine (abbreviation: VOPc) can be used. Also, thesubstance having the electron accepting property with respect to thesubstance with the hole transporting property is not particularlylimited. For example, molybdenum oxide, vanadium oxide,7,7,8,8-tetracyanoquinodimethane (abbreviation: TCNQ),2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (abbreviation:F4-TCNQ), and the like can be used. The first layer 111 preferablyincludes the substance having the electron accepting property withrespect to the substance with the hole transporting property so as tosatisfy a molar ratio (i.e., the substance having the electron acceptingproperty with respect to the substance with the hole transportingproperty/the substance with the hole transporting property) of 0.5 to 2.In addition, the first layer 111 may include a p-type semiconductor suchas molybdenum oxide, vanadium oxide, ruthenium oxide, cobalt oxide andcopper oxide.

The second layer 112 generates electrons. As the second layer 112, forexample, a layer including a substance with an electron transportingproperty and a substance having an electron donating property withrespect to the substance with the electron transporting property can begiven. The substance with the electron transporting property is asubstance of which an electron transporting property is stronger than ahole transporting property. The substance with the electron transportingproperty is not particularly limited. For example, a metal complex suchas tris(8-quinolinolato)aluminum (abbreviation: Alq₃),tris(4-methyl-8-quinolinolato)aluminum (abbreviation: Almq₃),bis(10-hydroxybenzo[h]quinolinato)beryllium (abbreviation: BeBq₂),bis(2-methyl-8-quinolinolato)-4-phenylphenolate-aluminum (abbreviation:BAlq), bis(2-[2-hydroxyphenyl]benzoxazolate)zinc (abbreviation:Zn(BOX)₂), bis(2-[2-hydroxyphenyl]benzothiazolate)zinc (abbreviation:Zn(BTZ)₂) can be used. In addition, the following substances can be usedas the substance with the electron transporting property:2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (abbreviation:PBD); 1,3-bis(5-[p-tert-butylphenyl]-1,3,4-oxadiazole-2-yl)benzene(abbreviation: OXD-7);3-(4-tert-butylphenyl)-4-phenyl-5-(4-biphenylyl)-1,2,4-triazole(abbreviation: TAZ);3-(4-tert-butylphenyl)-4-(4-ethylphenyl)-5-(4-biphenylyl)-1,2,4-triazole(abbreviation: p-EtTAZ); bathophenanthroline (abbreviation: BPhen);bathocuproin (abbreviation: BCP); and the like. Further, the substancehaving the electron donating property with respect to the substance withthe electron transporting property is not particularly limited. Forexample, alkali metal such as lithium and cesium, alkali earth metalsuch as magnesium and calcium, rare-earth metal such as erbium andytterbium, and the like can be used as the substance having the electrondonating property with respect to the substance with the electrontransporting property. Preferably, the second layer 112 includes thesubstance having the electron donating property with respect to thesubstance with the electron transporting property and the electrontransporting property so as to satisfy a molar ratio (i.e., thesubstance having the electron donating property with respect to thesubstance with the electron transporting property/the substance with theelectron transporting property) of 0.5 to 2. Additionally, the secondlayer 112 may include an n-type semiconductor such as zinc oxide, zincsulfide, zinc selenide, tin oxide and titanium oxide.

The third layer 113 contains a light emitting layer. The layer structureof the third layer 113 is not particularly limited. The third layer 113may include either a single layer or multiple layers. For example, asshown in FIG. 1, the third layer 113 may include an electrontransporting layer 121, a hole transporting layer 123 and a holeinjecting layer 124 along with the light emitting layer 122.Alternatively, the third layer 113 may include only the light emittinglayer.

The light emitting layer 122 contains a light emitting substance. Thelight emitting substance indicates a substance that can emit light witha desired wavelength and has an excellent light emitting efficiency. Thethird layer 113 is not particularly limited. The third layer 113 ispreferably formed using a layer in which a light emitting substance isdispersed and which is made from a substance having a larger energy gapthan that of the light emitting substance. Accordingly, light emittedfrom the light emitting substance can be prevented from going out due tothe concentration of the light emitting substance. Further, the energygap indicates an energy gap between the LUMO level and the HOMO level.

The light emitting substance is not particularly limited. A substancecapable of emitting light with a desired wavelength and having anexcellent light emitting efficiency may be used. In order to obtain redlight emission, for example, the following substances exhibitingemission spectrum with peaks at 600 to 680 nm can be employed:4-dicyanomethylene-2-isopropyl-6-(2-[1,1,7,7-tetramethyljulolidine-9-yl]ethenyl)-4H-pyran(abbreviation: DCJTI);4-dicyanomethylene-2-methyl-6-(2-[1,1,7,7-tetramethyljulolidine-9-yl]ethenyl)-4H-pyran(abbreviation: DCJT);4-dicyanomethylene-2-tert-butyl-6-(2-[1,1,7,7-tetramethyljulolidine-9-yl]ethenyl)-4H-pyran(abbreviation: DCJTB); periflanthene;2,5-dicyano-1,4-bis(2-[10-methoxy-1,1,7,7-tetramethyljulolidine-9-yl]ethenyl)benzeneand the like. In order to obtain green light emission, substancesexhibiting emission spectrum with peaks at 500 to 550 nm such asN,N′-dimethylquinacridon (abbreviation: DMQd), coumarin 6, coumarin545T, and tris(8-quinolinolate)aluminum (abbreviation: Alq₃) can beemployed. In order to obtain blue light emission, the followingsubstances exhibiting emission spectrum with peaks at 420 to 500 nm canbe employed: 9,10-bis(2-naphthyl)-tert-butylanthracene (abbreviation:t-BuDNA); 9,9′-bianthryl; 9,10-diphenylanthracene (abbreviation: DPA);9,10-bis(2-naphthyl)anthracene (abbreviation: DNA);bis(2-methyl-8-quinolinolate)-4-phenylphenolate-gallium (abbreviation:BGaq); bis(2-methyl-8-quinolinolate)-4-phenylphenolate-aluminum(abbreviation: BAlq); and the like.

A substance used for dispersing a light emitting substance is notparticularly limited. For example, an anthracene derivative such as9,10-di(2-naphthyl)-2-tert-butylanthracene (abbreviation: t-BuDNA), acarbazole derivative such as 4,4′-bis(N-carbazolyl)biphenyl(abbreviation: CBP), a metal complex such asbis(2-[2-hydroxyphenyl]pyridinato)zinc (abbreviation: Znpp₂) andbis(2-[2-hydroxyphenyl]benzoxazolato)zinc (abbreviation: ZnBOX), and thelike can be used.

In the above-described light emitting element, the difference inelectron affinity between the substance with the electron transportingproperty, which is included in the second layer 112 and a substance,which is included in one layer contacting to the second layer 112 amongthe layers included in the third layer 113, is preferably set to be 2 eVor less, more preferably, 1.5 eV or less. When the second layer 112 ismade by using an n-type semiconductor, the difference between a workfunction of the n-type semiconductor and the electron affinity of thesubstance, which is included in the layer contacting to the second layer112 among the layers included in the third layer 113, is preferably setto be 2 eV or less, more preferably, 1.5 eV or less.

Further, the layer contacting to the second layer 112 among the layersincluded in the third layer 113 corresponds to the electron transportinglayer 121 in the case where the third layer 113 comprises the structureof the present embodiment mode. When the third layer 113 includes onlythe light emitting layer, or, when the third layer 113 does not includethe electron transporting layer 121 or the like, the light emittinglayer corresponds to this layer contacting to the second layer 112. Inthe case where the light emitting layer is in contact with the secondlayer 112, a substance that is included in the layer contacting to thesecond layer 112 among the layers included in the third layer 113corresponds to a substance for dispersing the light emitting substanceor the light emitting substance itself. This is because, with respect toa light emitting substance like Alq₃ that can emit light without beingdispersed in the substance for dispersing the light-emitting substanceand has an excellent carrier transporting property, a layer made fromonly the light emitting substance can function as a light emitting layerwithout dispersing the light emitting substance in the substance fordispersing the light-emitting substance. Therefore, by contacting thethird layer 113 to the second layer 112, electrons can easily beinjected into the third layer 113 from the second layer 112.

Preferably, one or both of the first electrode 101 and the secondelectrode 102 is/are formed by using a conductive substance capable oftransmitting visible light. Accordingly, light generated in the lightemitting layer can be emitted to the outside through at least one of thefirst electrode 101 and the second electrode 102.

The first electrode 101 is not particularly limited. For example,aluminum, indium tin oxide (ITO), indium tin oxide containing siliconoxide, indium oxide containing 2 to 20% zinc oxide can be used as thefirst electrode. Additionally, gold (Au), platinum (Pt), nickel (Ni),tungsten (W), chromium (Cr), molybdenum (Mo), iron (Fe), cobalt (Co),copper (Cu), palladium (Pd) and the like can be used.

Also, the second electrode 102 is not particularly limited. When thesecond electrode 102 has a function of injecting holes to the thirdlayer 113 like the light emitting element of the present embodimentmode, the second electrode 102 is preferably made from a substancehaving a large work function. Concretely, indium tin oxide (ITO), indiumtin oxide containing silicon oxide, indium oxide containing 2 to 20%zinc oxide, gold (Au), platinum (Pt), nickel (Ni), tungsten (W),chromium (Cr), molybdenum (Mo), iron (Fe), cobalt (Co), copper (Cu),palladium (Pd) and the like can be used. Further, for instance, thesecond electrode 102 can be formed by sputtering, evaporation, or thelike.

As described above, the electron transporting layer 121 is sandwichedbetween the second layer 112 and the light emitting layer 122 in thepresent embodiment mode. The electron transporting layer 121 has afunction of transporting electrons injected therein to the lightemitting layer 122. By providing the electron transporting layer 121therebetween to isolate the first electrode 101 and the second layer 112containing metal from the light emitting layer 122, light generated inthe light emitting layer can be prevented from going out due to themetal.

The electron transporting layer 121 is not particularly limited and canbe formed by using the above-described Alq₃, Almq₃, BeBq₂, BAlq,Zn(BOX)₂, Zn(BTZ)₂, PBD, OXD-7, TAZ, p-EtTAZ, BPhen, BCP, or the like.The electron transporting layer 121 is preferably formed by using theabove-mentioned substance with an electron transporting property ofwhich the electron mobility is higher than the hole mobility. Also, theelectron transporting layer 121 is preferably formed by using asubstance having the electron mobility of 10⁻⁶ cm²/Vs or more. Further,the electron transporting layer 121 may have a multilayer structureformed by laminating two or more layers made from the above-describedsubstances.

In this embodiment mode, a hole transporting layer 123 is providedbetween the second electrode 102 and the light emitting layer 122 asshown in FIG. 1. The hole transporting layer 123 has a function oftransporting the holes injected from the second electrode 102 to thelight emitting layer 122. By providing the hole transporting layer 123to isolate the second electrode 102 from the light emitting layer 122,light generated in the light emitting layer can be prevented from goingout due to the metal.

The hole transporting layer 123 is not particularly limited. Theabove-described NPB, TPD, TDATA, MTDATA, DNTPD and the like can be usedas the hole transporting layer. Preferably, the hole transporting layer123 is formed by using the above-described substance with a holetransporting property of which the hole mobility is higher than theelectron mobility. Also, the hole transporting layer 123 is preferablyformed using a substance having the hole mobility of 10⁻⁶ cm²/Vs ormore. The hole transporting layer 123 may have a multilayer structureformed by laminating two or more layers made from the above-describedsubstances.

As shown in FIG. 1, the hole injecting layer 124 may be provided betweenthe second electrode 102 and the hole transporting layer 123. The holeinjecting layer 124 has a function of helping the injection of holesinto the hole transporting layer 123 from the second electrode 102.

The hole injecting layer 124 is not particularly limited. The holeinjecting layer can be formed by using metal oxide such as molybdenumoxide, vanadium oxide, ruthenium oxide, tungsten oxide and manganeseoxide. In addition, the hole injecting layer 124 can be formed by usingthe above-described phthalocyanine compound such as H₂Pc, CuPC and VOPc,the aromatic amine compound such as DNTPD, or a high molecular weightmaterial such as a poly(ethylenedioxythiophene)/poly(styrene sulfonate)mixture (PEDOT/PSS). Furthermore, the hole injecting layer 124 may beformed using the above-described layer including the substance with thehole transporting property and the substance having the electronaccepting property with respect to the substance with the holetransporting property.

The above-described light emitting element of the present invention is ahighly-reliable light emitting element in which the driving voltage isslightly increased with the accumulation of light emitting time.Further, the voltage applied to obtain the predetermined luminance isreferred to as the driving voltage here.

The light emitting element of the present invention has slight change involtage, which is applied to the light emitting element to flow thepredetermined current through the light emitting element, depending onthe thickness of the layer generating the holes (i.e., the first layer111). Therefore, for example, by increasing the thickness of the firstlayer 111 to increase the distance between the first and secondelectrodes, the first electrode 101 can be easily prevented fromshort-circuiting with the second electrode 102.

Embodiment Mode 2

This embodiment mode will describe a light emitting element in which alight extraction efficiency is increased by controlling the thickness ofa layer generating holes and an optical distance between an reflectingsurface and a light emitting region is controlled to reduce the changein emission spectrum depending on an angle of seeing the light emittingsurface, with reference to FIG. 24.

A light emitting element of FIG. 24 comprises a first layer 211generating holes, a second layer 212 generating electrons, and a thirdlayer 213 containing a light emitting substance between a firstelectrode 201 and a second electrode 202. The first layer 211, thesecond layer 212 and the third layer 213 are laminated to one anotherwhile sandwiching the second layer 212 between the first and thirdlayers. The first layer 211 is in contact with the first electrode 201while the third layer 213 is in contact with the second electrode 202.

The first electrode 201 is an electrode made from a conductive materialhaving high reflectance, or, a reflecting electrode. As the conductivematerial having the high reflectance, aluminum, silver, an alloy ofthese metals (e.g., an Al:Li alloy, an Mg:Ag alloy etc.) and the likecan be used. The conductive material preferably has the reflectance of50 to 100%. The second electrode 202 is made from a conductive materialthat can transmit visible light. The conductive material that cantransmit visible light is not particularly limited, and indium tinoxide, indium tin oxide containing silicon oxide, indium oxidecontaining 2 to 20% zinc oxide, or the like can be used.

When applying the voltage to the light emitting element such that apotential of the second electrode 202 is higher than that of the firstelectrode 201, holes are injected into the first electrode 201 from thefirst layer 211 while electrons are injected into the third layer 213from the second layer 212. Also, holes are injected into the third layer213 from the second electrode 202.

The electrons and holes are recombined in the third layer 213 so that alight emitting substance is excited. The light emitting substance emitslight upon retuning to the ground state from the excited state. A regionin which light is generated in this way is particularly referred to as alight emitting region. A layer including a light emitting substance forforming the light emitting region is referred to as a light emittinglayer. Further, the light emitting region is formed at least in a partof the light emitting layer.

In the light emitting element according to the present embodiment mode,the third layer 213 includes an electron transporting layer 221, a holetransporting layer 223 and a hole injecting layer 224, along with thelight emitting layer 222. Further, the structure of the third layer 213is not limited to the one shown in FIG. 24. For instance, the thirdlayer 213 may have a single layer structure including only the lightemitting layer.

The first layer 211, the second layer 212 and the third layer 213 may beformed by using the same materials of the first layer 111, the secondlayer 112 and the third layer 113 as described in Embodiment Mode 1,respectively. Similarly, the electron transporting layer 221, the lightemitting layer 222, the hole transporting layer 223 and the holeinjecting layer 224 may be formed by using the same materials of theelectron transporting layer 121, the light emitting layer 122, the holetransporting layer 123 and the hole injecting layer 124 as described inEmbodiment Mode 1, respectively.

When light entering into the reflecting electrode, a phase inversion iscaused in the reflected light. By the effect of interference of lightdue to the phase inversion, when an optical distance between the lightemitting region and the reflecting electrode (i.e.,reflectance×distance) is (2m−1)/4 times (m is a given positive integer)of the emission wavelength, or, when the optical distance is 1/4, 3/4,5/4 . . . times of the emission wavelength, the light extractionefficiency is increased. Meanwhile, when the optical distancetherebetween is m/2 times (m is a given positive integer), or, 1/2, 1,3/2 . . . times of the emission wavelength, the light extractionefficiency is reduced.

Therefore, in the case where the light emitting region is placed in thevicinity of an interface between the light emitting layer 222 and thehole transporting layer 223 in the light emitting element according tothe present embodiment mode, the respective thicknesses of the firstlayer 211, the second layer 212, the electron transporting layer 221 andthe light emitting layer 222 are preferably adjusted so as to satisfythe following expression 4. Accordingly, light can be emitted to theoutside efficiently. Also, the increase in resistance value with theincrease of film thicknesses of d_(i) and d_(ii) can be suppressed. Herethe resistance value indicates a value obtained by dividing the appliedvoltage (V) by the current (mA) flowing through the light emittingelement according to the applied voltage.

$\begin{matrix}{{{n_{i}d_{i}} + {n_{ii}d_{ii}} + {n_{1}d_{1}} + {n_{p}d_{p}}} = \frac{\left( {{2\; m} - 1} \right)\lambda}{4}} & 4\end{matrix}$

In the expression 4, n_(i) represents the refractive index of the firstlayer 211; d_(i), the thickness of the first layer 211; n_(ii), therefractive index of the second layer 212; d_(ii), the thickness of thesecond layer 212; n₁, the refractive index of the electron transportinglayer 221; d₁, the thickness of the electron transporting layer 221;n_(p), the refractive index of the light emitting layer 222; d_(p), thethickness of the light emitting layer 222; λ, the wavelength of lightgenerated in the light emitting element; and m, a given positiveinteger.

Meanwhile, in the case where the light emitting region is placed in thevicinity of an interface between the light emitting layer 222 and theelectron transporting layer 221 in the light emitting element of thepresent embodiment mode, the respective thicknesses of the first layer211, the second layer 212 and the electron transporting layer 221 arepreferably adjusted so as to satisfy the expression 5. Accordingly,light can be emitted to the outside portion efficiently. In addition,the increase in the resistance value with the increase in filmthicknesses of d_(i) and d_(ii) can be suppressed.

$\begin{matrix}{{{n_{i}d_{i}} + {n_{ii}d_{ii}} + {n_{1}d_{1}}} = \frac{\left( {{2\; m} - 1} \right)\lambda}{4}} & 5\end{matrix}$

In the expression 5, n_(i) represents the refractive index of the firstlayer 211; d_(i), the thickness of the first layer 211; n_(ii), therefractive index of the second layer 212; d_(ii), the thickness of thesecond layer 212; n₁, the refractive index of the electron transportinglayer 221; d₁, the thickness of the electron transporting layer 221; λ,the wavelength of light generated in the light emitting element; and m,a given positive integer.

Further, when the light emitting region is formed in the entire area ofthe light emitting layer 222 in the light emitting element of thisembodiment mode, the respective thicknesses of the first layer 211, thesecond layer 212 and the electron transporting layer 221 are preferablyadjusted so as to satisfy the following expression 6. Accordingly, lightcan be emitted to the outside efficiently.

$\begin{matrix}{{\frac{\left( {{2\; m} - 1} \right)\lambda}{4} - {n_{ii}d_{ii}} - {n_{1}d_{1}} - {n_{p}d_{p}}} \leqq {n_{i}d_{i}} \leqq {\frac{\left( {{2\; m} - 1} \right)\lambda}{4} - {n_{ii}d_{ii}} - {n_{1}d_{1}}}} & 6\end{matrix}$

In the expression 6, n_(i) represents the refractive index of the firstlayer 211; d_(i), the thickness of the first layer 211; n_(ii), therefractive index of the second layer 212; d_(ii), the thickness of thesecond layer 212; n₁, the refractive index of the electron transportinglayer 221; d₁, the thickness of the electron transporting layer 221;n_(p), the refractive index of the light emitting layer 222; d_(p), thethickness of the light emitting layer 222; λ, the wavelength of lightgenerated in the light emitting element; and m, a given positiveinteger.

In the expressions 4, 5 and 6, m preferably satisfies the relation of1≦m≦10. Concretely, the light generated in the light emitting elementindicates light emitted from the light emitting substance to the outsideof the light emitting element. Also, the wavelength of light emissionindicates a theoretical figure with respect to a wavelength showing amaximal value in emission spectrum.

When the first layer 211 is formed using a substance with a holetransporting property and the second layer 212 is formed using asubstance with an electron transporting property, in particular, d_(ii)is preferably equal to or greater than d_(i) in the above-mentionedexpressions 4, 5 and 6. Accordingly, the increase in the resistancevalue with the increase in film thickness can be further suppressed.This is because, in particular, a large amount of substance with thehole transporting property exists relative to the substance with theelectron transporting property in organic materials, and the substancewith the hole transporting property that has higher hole mobility iseasily 6512, a writing gate signal line driver circuit 6513 and anerasing gate signal line driver circuit 6514 are provided over asubstrate 6500. The source signal line driver circuit 6512, the writinggate signal line driver circuit 6513 and the erasing gate signal linedriver circuit. 6514 are connected to FPCs (flexible printed circuits)6503, which are external input terminals, through wiring groups,respectively. The source signal line driver circuit 6512, the writinggate signal line driver circuit 6513 and the erasing gate signal linedriver circuit 6514 receive video signals, clock signals, start signals,reset signals and the like from the FPCs 6503, respectively. The FPCs6503 are attached with printed wiring boards (PWBs) 6504. Further,driver circuits are not necessary to be formed over the same substrateas the pixel portion 6511. For example, the driver circuits may beprovided outside of the substrate by utilizing TCP in which an IC chipis mounted over an FPC having a wiring pattern, or the like.

A plurality of source signal lines extending in columns are aligned inrows in the pixel portion 6511. Also, power supply lines are aligned inrows. A plurality of gate signal lines extending in rows are aligned incolumns in the pixel portion 6511. In addition, a plurality of circuitseach including a light emitting element are aligned in the pixel portion6511.

FIG. 4 is a diagram showing a circuit for operating one pixel. Thecircuit as shown in FIG. 4 comprises a first transistor 901, a secondtransistor 902 and a light emitting element 903.

Each of the first and second transistors 901 and 902 is a three terminalelement including a gate electrode, a drain region and a source region.A channel region is interposed between the drain region and the sourceregion. The region serving as the source region and the region servingas the drain region are changed depending on a structure of atransistor, an operational condition and the like, and therefore, it isdifficult to determine which regions serve as the source region and thedrain region. Accordingly, the regions serving as the source and thedrain are denoted as a first electrode and a second electrode of eachtransistor in this embodiment mode, respectively.

A gate signal line 911 and a writing gate signal line driver circuit 913are obtained as compared with the substance with the electrontransporting property that has the higher electron mobility. Therefore,the light emitting element of the present invention can utilize thesubstance with the hole transporting property effectively. By utilizingthe substance with the hole transporting property effectively, the rangeof choices for materials that are used for forming the light emittingelement is widened, and hence, the light emitting element can be formedeasily.

The light emitting element having the structure in which the electrontransporting layer 221 is sandwiched between the second layer 212 andthe light emitting layer 222 is explained in this embodiment mode.Alternatively, the light emitting element may include a different layerbetween the second layer 212 and the light emitting layer 222, ratherthan the electron transporting layer 221. In this case, n₁d₁ in theexpression 6 can be expressed as follows: n₁d₁+n₂d₂ . . . +n_(k)d_(k)+ .. . .

Embodiment Mode 3

The light emitting element according to the present invention is ahighly reliable element having slight increase in the driving voltagewith the accumulation of light emitting time. By applying the lightemitting element according to the invention to, e.g., a pixel portion, alight emitting device having low power consumption can be obtained.Also, the light emitting element of the invention can prevent theshort-circuiting between electrodes easily. Therefore, by applying thelight emitting element of the invention to a pixel portion, a lightemitting device capable of displaying favorable images having lessdefects due to the short-circuiting can be obtained. Furthermore, thelight emitting element according to the invention can easily emit lightto the outside. By applying the light emitting element of the inventionto a pixel portion, a light emitting device capable of performingdisplay operation at low power consumption can be obtained.

In this embodiment mode, circuit structures and driving methods of alight emitting device having a display function will be described withreference to FIGS. 3, 4, 5 and 6.

FIG. 3 is a schematic top view of a light emitting device according tothe present invention. In FIG. 3, a pixel portion 6511, a source signalline driver circuit provided to be electrically connected ordisconnected to each other by a switch 918. The gate signal line 911 andan erasing gate signal line driver circuit 914 are provided to beelectrically connected or disconnected to each other by a switch 919. Asource signal line 912 is provided to be electrically connected toeither a source signal line driver circuit 915 or a power source 916 bya switch 920. A gate of the first transistor 901 is electricallyconnected to the gate signal line 911. The first electrode of the firsttransistor 901 is electrically connected to the source signal line 912while the second electrode thereof is electrically connected to a gateelectrode of the second transistor 902. The first electrode of thesecond transistor 902 is electrically connected to a current supply line917 while the second electrode thereof is electrically connected to oneelectrode included in a light emitting element 903. Further, the switch918 may be included in the writing gate signal line driver circuit 913.The switch 919 may also be included in the erasing gate signal linedriver circuit 914. In addition, the switch 920 may be included in thesource signal line driver circuit 915.

The arrangement of transistors, light emitting elements and the like inthe pixel portion is not particularly limited. For example, thearrangement as shown in a top view of FIG. 5 can be employed. In FIG. 5,a first electrode of a first transistor 1001 is connected to a sourcesignal line 1004 while a second electrode of the first transistor isconnected to a gate electrode of a second transistor 1002. A firstelectrode of the second transistor 1002 is connected to a current supplyline 1005 and a second electrode of the second transistor is connectedto an electrode 1006 of a light emitting element. A part of the gatesignal line 1003 functions as a gate electrode of the first transistor1001.

Next, the method for driving the light emitting device will be describedbelow. FIG. 6 is a diagram explaining an operation of a frame with time.In FIG. 6, a horizontal direction indicates time passage while alongitudinal direction indicates the number of scanning stages of a gatesignal line.

When an image is displayed on the light emitting device according to theinvention, a rewriting operation and a displaying operation are carriedout during a display period, repeatedly. The number of rewritingoperations is not particularly limited. However, the rewriting operationis preferably performed about 60 times a second such that a person whowatches a displayed image does not detect flicker in the image. A periodof operating the rewriting operation and the displaying operation of oneimage (one frame) is, herein, referred to as one frame period.

As shown in FIG. 6, one frame is divided into four sub-frames 501, 502,503 and 504 including writing periods 501 a, 502 a, 503 a and 504 a andholding periods 501 b, 502 b, 503 b and 504 b. The light emittingelement applied with a signal for emitting light emits light during theholding periods. The length ratio of the holding periods in each of thefirst sub-frame 501, the second sub-frame 502, the third sub-frame 503and the fourth sub-frame 504 satisfies 2³:2²:2¹:2⁰=8:4:2:1. This allowsthe light emitting device to exhibit 4-bit gray scale. Further, thenumber of bits and the number of gray scales are not limited to those asshown in this embodiment mode. For instance, one frame may be dividedinto eight sub-frames so as to achieve 8-bit gray scale.

The operation in one frame will be described. In the sub-frame 501, thewriting operation is first performed in 1^(st) row to a last row,sequentially. Therefore, the starting time of the writing periods isvaried for each row. The holding period 501 b sequentially starts in therows in which the writing period 501 a has been terminated. In theholding period 501 b, a light emitting element applied with a signal foremitting light remains in a light emitting state. Upon terminating theholding period 501 b, the sub-frame 501 is changed to the next sub-frame502 sequentially in the rows. In the sub-frame 502, a writing operationis sequentially performed in the 1^(st) row to the last row in the samemanner as the sub-frame 501. The above-mentioned operations are carriedout repeatedly up to the holding period 504 b of the sub-frame 504 andthen terminated. After terminating the operation in the sub-frame 504,an operation in the next frame starts. Accordingly, the sum of thelight-emitting time in respective sub-frames corresponds to the lightemitting time of each light emitting element in one frame. By changingthe light emitting time for each light emitting element and combiningsuch the light emitting elements variously within one pixel, variousdisplay colors with different brightness and different chromaticity canbe formed.

When the holding period is intended to be forcibly terminated in the rowin which the writing period has already been terminated and the holdingperiod has started prior to terminating the writing operation up to thelast row as shown in the sub-frame 504, an erasing period 504 c ispreferably provided after the holding period 504 b so as to stop lightemission forcibly. The row where light emission is forcibly stopped doesnot emit light for a certain period (this period is referred to as a nonlight emitting period 504 d). Upon terminating the writing period in thelast row, a writing period of a next sub-frame (or, a next frame) startssequentially from a first row. This can prevent the writing period inthe sub-frame 504 from overlapping with the writing period in the nextsub-frame.

Although the sub-frames 501 to 504 are arranged in order of increasingthe length of the holding period in this embodiment mode, they are notnecessary to be arranged in this order. For example, the sub-frames maybe arranged in ascending order of the length of the holding period.Alternatively, the sub-frames may be arranged in random order. Inaddition, these sub-frames may further be divided into a plurality offrames. That is, scanning of gate signal lines may be performed atseveral times during a period of supplying same video signals.

The operations of the circuits in the writing period and the erasingperiod as shown in FIG. 4 will be described below.

The operation in the writing period will be described first. In thewriting period, the gate signal line 911 in the n^(th) row (n is anatural number) is electrically connected to the writing gate signalline driver circuit 913 via the switch 918. The gate signal line 911 inthe n^(th) row is not connected to the erasing gate signal line drivercircuit 914. The source signal line 912 is electrically connected to thesource signal line driver circuit 915 via the switch 920. In this case,a signal is input in a gate of the first transistor 901 connected to thegate signal line 911 in the n^(th) row (n is a natural number), therebyturning the first transistor 901 on. At this moment, video signals aresimultaneously input in the source signal lines in the first to lastcolumns. Further, the video signals input from the source signal line912 in each column are independent from one another. The video signalsinput from the source signal line 912 are input in a gate electrode ofthe second transistor 902 via the first transistor 901 connected to therespective source signal lines. At this moment, it is decided whetherthe light emitting element 903 emits light or emits no light dependingon the signals input in the second transistor 902. For instance, whenthe second transistor 902 is a P-channel type, the light emittingelement 903 emits light by inputting a low level signal in the gateelectrode of the second transistor 902. On the other hand, when thesecond transistor 902 is an N-channel type, the light emitting element903 emits light by inputting a high level signal in the gate electrodeof the second transistor 902.

Next, the operation in the erasing period will be described. In theerasing period, the gate signal line 911 in the n^(th) row (n is anatural number) is electrically connected to the erasing gate signalline driver circuit 914 via the switch 919. The gate signal line 911 inthe n^(th) row is not connected to the writing gate signal line drivercircuit 913. The source signal line 912 is electrically connected to thepower source 916 via the switch 920. In this case, upon inputting asignal in the gate of the first transistor 901 connecting to the gatesignal line 911 in the n^(th) row, the first transistor 901 is turnedon. At this moment, erasing signals are simultaneously input in thesource signal lines in the first to last columns. The erasing signalsinput from the source signal line 912 are input in the gate electrode ofthe second transistor 902 via the first transistor 901 connecting to therespective source signal lines. A supply of current flowing through thelight emitting element 903 from the current supply line 917 is forciblystopped by the signals input in the second transistor 902. This makesthe light emitting element 903 emit no light forcibly. For example, whenthe second transistor 902 is a P-channel type, the light emittingelement 903 emits no light by inputting a high level signal in the gateelectrode of the second transistor 902. On the other hand, when thesecond transistor 902 is an N-channel type, the light emitting element903 emits no light by inputting a low level signal in the gate electrodeof the second transistor 902.

Further, in the erasing period, a signal for erasing is input in then^(th) row (n is a natural number) by the above-mentioned operation.However, as mentioned above, the n^(th) row sometimes remains in theerasing period while another row (e.g., a m^(th) row (m is a naturalnumber)) remains in the writing period. In this case, since a signal forerasing is necessary to be input in the n^(th) row and a signal forwriting is necessary to be input in the m^(th) row by utilizing thesource signal lines in the same columns, the after-mentioned operationis preferably carried out.

After the light emitting element 903 in the n^(th) row becomes anon-light emitting state by the above-described operation in the erasingperiod, the gate signal line 911 and the erasing gate signal line drivercircuit 914 are immediately disconnected to each other and the sourcesignal line 912 is connected to the source signal line driver circuit915 by turning the switch 920 on/off. The gate signal line 911 and thewriting gate signal line driver circuit 913 are connected to each otherwhile the source signal line and the source signal line driver circuit915 are connected to each other. A signal is selectively input in thesignal line in the m^(th) row from the writing gate signal line drivercircuit 913 and the first transistor is turned on while signals forwriting are input in the source signal lines in the first to lastcolumns from the source signal line driver circuit 915. By inputtingthese signals, the light emitting element in the m^(th) row emits lightor no light.

After terminating the writing period in the m^(th) row as mentionedabove, the erasing period immediately starts in the n+1^(th) row.Therefore, the gate signal line 911 and the writing gate signal linedriver circuit 913 are disconnected to each other and the source signalline is connected to the power source 916 by turning the switch 920on/off. Also, the gate signal line 911 and the writing gate signal linedriver circuit 913 are disconnected to each other and the gate signalline 911 is connected to the erasing gate signal line driver circuit914. A signal is selectively input in the gate signal line in then+1^(th) row from the erasing gate signal line driver circuit 914 andthe first transistor is turn on while an erasing signal is input thereinfrom the power source 916. Upon terminating the erasing period in then+1^(th) row in this manner, the writing period immediately starts inthe m+1^(th) row. The erasing period and the writing period may berepeated alternatively until the erasing period of the last row.

Although the writing period of the m^(th) row is provided between theerasing period of the n^(th) row and the erasing period of the n+1^(th)row in this embodiment mode, the present invention is not limitedthereto. The writing period of the m^(th) row may be provided betweenthe erasing period in the n−1^(th) row and the erasing period in then^(th) row.

Furthermore, in this embodiment mode, when the non-light emitting period504 d is provided like the sub-frame 504, the operation of disconnectingthe erasing gate signal line driver circuit 914 from one gate signalline while connecting the writing gate signal line driver circuit 913 toanother gate signal line is carried out repeatedly. This operation maybe performed in a frame in which a non-light emitting period is notparticularly provided.

Embodiment Mode 4

An example of a cross sectional view of a light emitting deviceincluding a light emitting element according to the invention will bedescribed with reference to FIGS. 7A to 7C.

In each of FIGS. 7A to 7C, a region surrounded by a dashed linerepresents a transistor 11 that is provided for driving a light emittingelement 12 of the invention. The light emitting element 12 of theinvention comprises a layer 15 in which a lamination of a layergenerating holes, a layer generating electrons and a layer including alight emitting substance is provided between a first electrode 13 and asecond electrode 14. A drain of the transistor 11 and the firstelectrode 13 are electrically connected to each other by a wiring 17that passes through a first interlayer insulating film 16 (16 a, 16 band 16 c). The light emitting element 12 is isolated from another lightemitting elements provided adjacent to the light emitting element 12 bya partition wall layer 18. The light emitting device of the inventionhaving this structure is provided over a substrate 10 in this embodimentmode.

The transistor 11 as shown in each FIGS. 7A to 7C is a top-gate typetransistor in which a gate electrode is provided on a side of asemiconductor layer opposite to the substrate. Further, the structure ofthe transistor 11 is not particularly limited. For example, abottom-gate type transistor may be employed. In the case of using abottom-gate type transistor, either a transistor in which a protectionfilm is formed on a semiconductor layer of a channel (a channelprotection type transistor) or a transistor in which a part of asemiconductor layer of a channel is etched (a channel etched typetransistor) may be used.

The semiconductor layer included in the transistor 11 may be any of acrystalline semiconductor, an amorphous semiconductor, a semiamorphoussemiconductor, and the like.

Concretely, the semiamorphous semiconductor has an intermediatestructure between an amorphous structure and a crystalline structure(including a single crystalline structure and a polycrystallinestructure), and a third condition that is stable in term of free energy.The semiamorphous semiconductor further includes a crystalline regionhaving a short range order along with lattice distortion. A crystalgrain with a size of 0.5 to 20 nm is included in at least a part of ansemiamorphous semiconductor film. Raman spectrum is shifted toward lowerwavenumbers than 520 cm⁻¹. The diffraction peaks of (111) and (220),which are believed to be derived from Si crystal lattice, are observedin the semiamorphous semiconductor by the X-ray diffraction. Thesemiamorphous semiconductor contains hydrogen or halogen of at least 1atom % or more for terminating dangling bonds. The semiamorphoussemiconductor is also referred to as a microcrystalline semiconductor.The semiamorphous semiconductor is formed by glow dischargedecomposition with silicide gas (plasma CVD). As for the silicide gas,SiH₄, Si₂H₆, SiH₂Cl₂, SiHCl₃, SiCl₄, SiF₄ and the like can be used. Thesilicide gas may also be diluted with H₂, or a mixture of H₂ and one ormore of rare gas elements selected from He, Ar, Kr and Ne. The dilutionratio is set to be in the range of 1:2 to 1:1,000. The pressure is setto be approximately in the range of 0.1 to 133 Pa. The power frequencyis set to be 1 to 120 MHz, preferably, 13 to 60 MHz. The substrateheating temperature may be set to be 300° C. or less, more preferably,100 to 250° C. With respect to impurity elements contained in the film,each concentration of impurities for atmospheric constituents such asoxygen, nitrogen and carbon is preferably set to be 1×10²⁰/cm³ or less.In particular, the oxygen concentration is set to be 5×10¹⁹/cm³ or less,preferably, 1×10¹⁹/cm³ or less.

As a specific example of a crystalline semiconductor layer, asemiconductor layer made from single crystalline silicon,polycrystalline silicon, silicon germanium, or the like can be cited.These materials may be formed by laser crystallization. For example,these materials may be formed by crystallization with use of the solidphase growth method using nickel and the like.

When a semiconductor layer is made from an amorphous substance, e.g.,amorphous silicon, it is preferable to use a light emitting device withcircuits including only N-channel transistors as the transistor 11 andother transistor (a transistor included in a circuit for driving a lightemitting element). Alternatively, a light emitting device with circuitsincluding either N-channel transistors or P-channel transistors may beemployed. Also, a light emitting device with circuits including both anN-channel transistor and a P-channel transistor may be used.

The first interlayer insulating film 16 may include plural layers (e.g.,interlayer insulating films 16 a, 16 b and 16 c) as shown in FIGS. 7A to7C or a single layer. The interlayer insulating film 16 a is made froman inorganic material such as silicon oxide and silicon nitride. Theinterlayer insulating film 16 b is made from acrylic, siloxane (which isa substance that has a skeleton structure formed by silicon (Si)-oxygen(O) bonds and includes an organic group such as an alkyl group as itssubstituent), or a substance with a self-planarizing property that canbe formed by applying a liquid such as silicon oxide. The interlayerinsulating film 16 c is made from a silicon nitride film containingargon (Ar). The substances constituting the respective layers are notparticularly limited thereto. Therefore, substances other than theabove-mentioned substances may be employed. Alternatively, theabove-mentioned substances may be used in combination with a substanceother than the above-mentioned substances. Accordingly, the firstinterlayer insulating film 16 may be formed by using both an inorganicmaterial and an organic material or by using either an inorganicmaterial or an organic material.

The edge portion of the partition wall layer 18 preferably has a shapein which the radius of curvature is continuously varied. This partitionwall layer 18 is formed by using acrylic, siloxane, resist, siliconoxide, and the like. Further, the partition wall layer 18 may be madefrom any one of or both an inorganic film and an organic film.

FIGS. 7A and 7C show the structures in which only the first interlayerinsulating films 16 are sandwiched between the transistors 11 and thelight emitting elements 12. Alternatively, as shown in FIG. 7B, thefirst interlayer insulating film 16 (16 a and 16 b) and a secondinterlayer insulting film 19 (19 a and 19 b) may be provided between thetransistor 11 and the light emitting element 12. In the light emittingdevice as shown in FIG. 7B, the first electrode 13 passes through thesecond interlayer insulating film 19 to be connected to the wiring 17.

The second interlayer insulating film 19 may include either plurallayers or a single layer as well as the first interlayer insulating film16. The interlayer insulating film 19 a is made from acrylic, siloxane,or a substance with a self-planarizing property that can be formed byapplying a liquid such as silicon oxide. The interlayer insulating film19 b is made from a silicon nitride film containing argon (Ar). Thesubstances constituting the respective interlayer insulating layers arenot particularly limited thereto. Therefore, substances other than theabove-mentioned substances may be employed. Alternatively, theabove-mentioned substances may be used in combination with a substanceother than the above-mentioned substances. Accordingly, the secondinterlayer insulating film 19 may be formed by using both an inorganicmaterial and an organic material or by using either an inorganicmaterial or an organic material.

When the first electrode and the second electrode are both formed byusing a substance with a light transmitting property in the lightemitting element 12, light generated in the light emitting element 12can be emitted through both the first electrode 13 and the secondelectrode 14 as shown in arrows in FIG. 7A. When only the secondelectrode 14 is made from a substance with a light transmittingproperty, light generated in the light emitting element 12 can beemitted only through the second electrode 14 as shown in an arrow ofFIG. 7B. In this case, the first electrode 13 is preferably made from amaterial with high reflectance or a film (reflection film) made from amaterial with high reflectance is preferably provided under the firstelectrode 13. When only the first electrode 13 is made from a substancewith a light transmitting property, light generated in the lightemitting element 12 can be emitted only through the first electrode 13as shown in an arrow of FIG. 7C. In this case, the second electrode 14is preferably made from a material with high reflectance or a reflectionfilm is preferably provided over the second electrode 14.

Moreover, the light emitting element 12 may be formed by laminating thelayer 15 that is operated in applying the voltage to the light emittingelement such that a potential of the second electrode 14 is higher thanthat of the first electrode 13. Alternatively, the light emittingelement 12 may be formed by laminating the layer 15 that is operated inapplying the voltage to the light emitting element such that a potentialof the second electrode 14 is lower than that of the first electrode 13.In the former case, the transistor 11 is an N-channel transistor. In thelatter case, the transistor 11 is a P-channel transistor.

As set forth above, an active light emitting device that controls thedriving of the light emitting element using the transistor is describedin this embodiment mode. In addition, a passive light emitting devicethat drives a light emitting element without providing a driving elementsuch as a transistor may be employed. In this passive light emittingdevice, it can be driven at low power consumption by using the lightemitting element of the invention that is operated at a low drivingvoltage.

Embodiment Mode 5

By mounting a light emitting device according to the present invention,an electronic appliance with a slight increase of power consumption in adisplay portion or the like can be obtained. Also, by mounting a lightemitting device of the invention, an electronic appliance such as adisplay device capable of displaying favorable images with few defectsin pixels and the like can be obtained. Furthermore, by mounting thelight emitting device of the invention, an electronic appliance havinglow power consumption can be obtained.

Examples of electronic appliances mounted with the light emittingdevices according to the invention are illustrated in FIGS. 8A to 8C.

FIG. 8A is a laptop personal computer manufactured according to theinvention, including a main body 5521, a housing 5522, a display portion5523, a keyboard 5524 and the like. The laptop personal computer can beachieved by incorporating the light emitting device including the lightemitting element of the invention thereinto as the display portion 5523.

FIG. 8B is a cellular phone manufactured according to the invention,including a main body 5552, a display portion 5551, an audio outputportion 5554, an audio input portion 5555, operation switches 5556 and5557, an antenna 5553 and the like. The cellular phone can be achievedby incorporating the light emitting device including the light emittingelement of the invention thereinto as the display portion 5551.

FIG. 8C is a television set manufactured according to the invention,including a display portion 5531, a housing 5532, speakers 5533 and thelike. The television set can be achieved by incorporating the lightemitting device including the light emitting element of the inventionthereinto s the display portion 5531.

As set forth above, the light emitting devices of the invention aresuitable to be used as the display portions of various kinds ofelectronic appliances.

Further, the light emitting devices having the light emitting elementsof the invention are mounted on the laptop personal computer, thecellular phone and the television set. However, the light emittingdevices having the light emitting elements of the invention can bemounted on a navigation system, a lighting appliance and the like.

Embodiment 1

Methods for manufacturing four light emitting elements (i.e., a lightemitting element 1, a light emitting element 2, a light emitting element3 and a light emitting element 4) each having a different mixture ratioof a substance with a hole transporting property to a substance havingan electron accepting property with respect to the substance with thehole transporting property in a layer having a function of generatingholes, and characteristics of these elements will be described in thisembodiment with reference to FIG. 2.

Indium tin oxide containing silicon was formed over a substrate 701 bysputtering to form a second electrode 702. The second electrode 702 wasformed to have a thickness of 110 nm. Further, a substrate made of glasswas used as the substrate 701.

Next, a layer 703 including molybdenum oxide was formed on the secondelectrode 702 by vacuum evaporation of the molybdenum oxide. The layer703 was formed to have a thickness of 5 nm.

Next, a layer 704 including4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB) wasformed on the layer 703 by vacuum evaporation of the NPB. The layer 704was formed to have a thickness of 55 nm.

A layer 705 including tris(8-quinolinolato)aluminum (abbreviation: Alq₃)and coumarin 6 was formed on the layer 704 by co-evaporation of the Alq₃and the coumarin 6. The Alq₃-coumarin 6 weight ratio was adjusted tosatisfy 1:0.005. Accordingly, the coumarin 6 is dispersed in Alq₃. Thethickness of the layer 705 was set to be 35 nm. Further, theco-evaporation is an evaporation method that is performed simultaneouslyfrom plural evaporation sources.

A layer 706 including Alq₃ was formed on the layer 705 by vacuumevaporation of the Alq₃. The thickness of the layer 706 was set to be 10nm.

Next, a second layer 707 including Alq₃ and lithium (Li) was formed onthe layer 706 by co-evaporation of the Alq₃ and the lithium. TheAlq₃-lithium weight ratio was adjusted to satisfy 1:0.01. Accordingly,the lithium is dispersed in the Alq₃. The thickness of the second layer707 was set to be 10 nm.

Subsequently, a first layer 708 including NPB and molybdenum oxide wasformed on the second layer 707 by co-evaporation of the NPB and themolybdenum oxide. At this moment, with respect to the light emittingelement 1, the molar ratio between the NPB and the molybdenum oxide(=molybdenum oxide/NPB) was adjusted to satisfy 0.5. With respect to thelight emitting element 2, the molar ratio between the NPB and themolybdenum oxide (=molybdenum oxide/NPB) was adjusted to satisfy 1.0.With respect to the light emitting element 3, the molar ratio betweenthe NPB and the molybdenum oxide (=molybdenum oxide/NPB) was adjusted tosatisfy 1.5. With respect to the light emitting element 4, the molarratio between the NPB and the molybdenum oxide (=molybdenum oxide/NPB)was adjusted to satisfy 2.0. The thicknesses of the second layers forthe respective light emitting elements were set to be 20 nm.

Next, a first electrode 709 was formed on the first layer 708 by vacuumevaporation of aluminum. The thickness of the first electrode was set tobe 100 nm.

When current flows through each light emitting element manufacturedabove by applying the voltage thereto such that a potential of thesecond electrode 702 is higher than that of the first electrode 709,holes generated in the first layer 708 are injected in the firstelectrode 709 while electrons generated in the second layer 707 areinjected in the layer 706. The holes are injected in the layer 703 fromthe second electrode 702. The holes injected from the second electrode702 and the electrons injected from the second layer 707 are recombinedin the layer 705, allowing the coumarin 6 to emit light. Accordingly,the layer 705 serves as a light emitting layer. Further, the layer 703serves as a hole injecting layer. The layer 704 serves as a holetransporting layer. The layer 706 serves as an electron transportinglayer. In each light emitting element of the present embodiment, thesubstances included in the layer 706 and a substance with an electrontransporting property included in the second layer 707 are both Alq₃ andhave equivalent electron affinity.

FIG. 9 shows the voltage-luminance characteristics of the light emittingelements according to the present embodiment, FIG. 10 shows the currentdensity-luminance characteristics thereof, and FIG. 11 shows thevoltage-current characteristics thereof. In FIG. 9, a horizontal axisrepresents the voltage (V) while a perpendicular axis represents theluminance (cd/m²). In FIG. 10, a horizontal axis represents the currentdensity (mA/cm²) while a perpendicular axis represents luminance(cd/m²). In FIG. 11, a horizontal axis represents the voltage (V) whilea perpendicular axis represents the current (mA). In FIGS. 9, 10 and 11,a curve marked by ▴ indicates the characteristics of the light emittingelement 1, a curve marked by ● indicates the characteristics of thelight emitting element 2, a curve marked by ∘ indicates thecharacteristics of the light emitting element 3, and a curve marked by ▪indicates the characteristics of the light emitting element 4.

According to FIGS. 9, 10 and 11, it is known that respective lightemitting elements are operated favorably. In particular, it is knownthat the light emitting elements 2, 3 and 4 have the higher luminance,which is obtained by applying the predetermined voltage to the lightemitting elements, and the larger amount of current, wherein the molarratios between the NPB and the molybdenum oxide (i.e., molybdenumoxide/NPB) of the respective first layers 708 satisfy 1 to 2.Accordingly, by adjusting the molar ratio between the NPB and themolybdenum oxide (i.e., molybdenum oxide/NPB) to satisfy 1 to 2, a lightemitting element capable of operating at low driving voltage can beobtained.

Next, results of carrying out a continuous lighting test using the lightemitting elements of the present embodiment will be described. Thecontinuous lighting test was performed as shown below at normaltemperature after sealing the above-manufactured light emitting elementsunder nitrogen atmosphere.

As shown in FIG. 10, the light emitting element of the present inventionunder an initial condition requires the current density of 26.75 mA/cm²to emit light at the luminance of 3,000 cd/m². In this embodiment, thechange in voltage with time and the change in luminance with time thatwere required for flowing the current of 26.75 mA/cm² were examinedwhile flowing the current of 26.75 mA/cm² for a certain period. Themeasurement results are shown in FIG. 12 and FIG. 13. In FIG. 12, ahorizontal axis represents time passage (hour) while a perpendicularaxis represents the voltage (V) required for flowing the current of26.75 mA/cm². Also, in FIG. 13, a horizontal axis represents timepassage (hour) while a perpendicular axis represents the luminance (at agiven unit). Further, the luminance (at a given unit) is a relativevalue with respect to the initial luminance (i.e., the luminance at agiven time is divided by the initial luminance and then multiplied by100), wherein the luminance in an initial condition is expressed as 100.

According to FIG. 12, it is known that the voltage required for flowingthe current with the current density of 26.75 mA/cm² is increased toonly about 1 V from the initial condition after a lapse of 100 hours.Consequently, it is known that the light emitting elements are favorableelements having slight rise in voltage with time passage.

Embodiment 2

A method for manufacturing a light emitting element of the presentinvention will be described below with reference to FIG. 14.

Indium tin oxide containing silicon was formed over a substrate 731 bysputtering to form a second electrode 732. The thickness of the secondelectrode 732 was set to be 110 nm. Further, a substrate made of glasswas used as the substrate 731.

Next, a layer 733 including molybdenum oxide and NPB was formed on thesecond electrode 732 by co-evaporation of the molybdenum oxide and theNPB. The thickness of the layer 733 was set to be 50 nm.

Subsequently, a layer 734 including NPB was formed on the layer 733 byvacuum evaporation of the NPB. The thickness of the layer 734 was set tobe 10 nm.

A layer 735 including tris(8-quinolinolato)aluminum (abbreviation: Alq₃)and coumarin 6 was formed on the layer 734 by co-evaporation of the Alq₃and the coumarin 6. The Alq₃-coumarin 6 weight ratio was adjusted tosatisfy 1:0.005 so that the coumarin 6 was dispersed in Alq₃. Thethickness of the layer 735 was set to be 35 nm. Further, theco-evaporation is an evaporation method that is performed simultaneouslyfrom plural evaporation sources.

A layer 736 including Alq₃ was formed on the layer 735 by vacuumevaporation of the Alq₃. The thickness of the layer 736 was set to be 10nm.

A second layer 737 including Alq₃ and lithium (Li) was formed on thelayer 736 by co-evaporation of the Alq₃ and the lithium. TheAlq₃-lithium weight ratio was adjusted to satisfy 1:0.01 so that thelithium was dispersed in the Alq₃. The thickness of the second layer 737was set to be 10 nm.

Next, a first layer 738 including NPB and molybdenum oxide was formed onthe second layer 737 by co-evaporation of the NPB and the molybdenumoxide. The molar ratio between the NPB and the molybdenum oxide (i.e.,molybdenum oxide/NPB) was adjusted to be 1.0. The thickness of the firstlayer 738 was set to be 20 nm.

A first electrode 739 was formed on the first layer 738 by vacuumevaporation of aluminum. The thickness of the first electrode 739 wasset to be 100 nm.

When the current flows through the above-manufactured light emittingelement by applying the voltage thereto such that a potential of thesecond electrode 732 is higher than that of the first electrode 739,holes generated in the first layer 738 are injected in the firstelectrode 739 while electrons generated in the second layer 737 areinjected in the layer 736. The holes are injected in the layer 733 fromthe second electrode 732. The holes injected from the second electrode732 and the electrons injected from the second layer 737 are recombinedin the layer 735, allowing the coumarin 6 to emit light. Accordingly,the layer 735 serves as a light emitting layer. Further, the layer 733serves as a hole injecting layer. The layer 734 serves as a holetransporting layer. The layer 736 serves as an electron transportinglayer. In the light emitting element according to the presentembodiment, the substances included in the layer 736 and a substancewith an electron transporting property included in the second layer 737are both Alq₃ and have equivalent electron affinity.

Comparative Example

Next, a method for manufacturing a light emitting element of thecomparative example will be described with reference to FIG. 15.

Indium tin oxide containing silicon was formed over a substrate 751 bysputtering to form a second electrode 752. The thickness of the secondelectrode 752 was set to be 110 nm. A substrate made of glass was usedas the substrate 751.

Next, a layer 753 including molybdenum oxide and NPB was formed on thesecond electrode 752 by co-evaporation of the molybdenum oxide and theNPB. The thickness of the layer 753 was set to be 50 nm.

A layer 754 including NPB was formed on the layer 753 by vacuumevaporation of the NPB. The thickness of the layer 754 was set to be 10nm.

A layer 755 including Alq₃ and coumarin 6 was formed on the layer 754 byco-evaporation of Alq₃ and coumarin 6. The Alq₃-coumarin 6 weight ratiowas adjusted to satisfy 1:0.005 so that the coumarin 6 was dispersed inAlq₃. The thickness of the layer 755 was set to be 35 nm.

A layer 756 including Alq₃ was formed on the layer 755 by vacuumevaporation of the Alq₃. The thickness of the layer 756 was set to be 10nm

Next, a second layer 757 including Alq₃ and lithium (Li) was formed onthe layer 756 by co-evaporation of the Alq₃ and the lithium. TheAlq₃-lithium weight ratio was adjusted to satisfy 1:0.01 so that thelithium was dispersed in the Alq₃. The thickness of the second layer 757was set to be 10 nm.

Next, a first electrode 758 was formed on the second layer 757 by vacuumevaporation of aluminum. The thickness of the first electrode 758 wasset to be 100 nm.

The light emitting element of the comparative example was manufacturedin the above-described manner to be compared with the light emittingelement of Embodiment 2 according to the present invention. As seen fromthe above, the light emitting element of the comparative example doesnot include a layer corresponding to the first layer 738 of Embodiment2.

The voltage-luminance characteristics of the light emitting element ofEmbodiment 2 and the light emitting element of the comparative exampleare shown in FIG. 16 while the voltage-current characteristics thereofare shown in FIG. 17. In FIG. 16, a horizontal axis represents thevoltage (V) and a perpendicular axis represents the luminance (cd/m²).In FIG. 17, a horizontal axis represents the voltage (V) and aperpendicular axis represents the current (mA). In FIG. 16 and FIG. 17,a curve marked by ● indicates the characteristics of the light emittingelement of Embodiment 2 (present invention) whereas a curve marked by ▴indicates the characteristics of the light emitting element of thecomparative example.

According to FIG. 16, it is known that the luminance of the lightemitting element of the present invention, which is obtained in applyingthe predetermined voltage thereto, is higher than that of the lightemitting element of the comparative example. In addition, it is knownthat the current flowing through the light emitting element of thepresent invention upon applying the predetermined voltage thereto ishigher than that of the light emitting element of the comparativeexample. Consequently, the light emitting element of the presentinvention is a favorable element capable of operating at low drivingvoltage.

Each of the light emitting elements as shown in Embodiment 1 andEmbodiment 2 comprises layers functioning as a hole injecting layer, ahole transporting layer, an electron transporting layer and the like,together with a layer functioning as a light emitting layer. However,these layers may not be necessary to be formed. Further, after formingthe layer functioning as the light emitting layer, the layer generatingelectrons is formed, followed by forming the layer generating holes inEmbodiment 1 and Embodiment 2. However, the method for manufacturing thelight emitting element according to the present invention is not limitedthereto. For example, after forming the layer generating the holes, thelayer generating electrons may be formed, followed by forming a layerincluding a layer functioning as a light emitting layer.

Embodiment 3

Methods for manufacturing six light emitting elements having differentthicknesses of layers generating holes (i.e., a light emitting element5, a light emitting element 6, a light emitting element 7, a lightemitting element 8, a light emitting element 9, a light emitting element10 and a light emitting element 11), and characteristics of theseelements will be described in this embodiment with reference to FIG. 18.

Indium tin oxide was formed over a substrate 771 by sputtering to form asecond electrode 772 with a thickness of 110 nm. A substrate made ofglass was used as the substrate 771.

A layer 773 including CuPC was formed on me second electrode 772 byvacuum evaporation of the CuPC. The thickness of the layer 773 was setto be 20 nm.

A layer 774 including NPB was next formed on the layer 773 by vacuumevaporation of the NPB. The thickness of the layer 774 was set to be 40nm.

Next, a layer 775 including Alq₃ and coumarin 6 was formed on the layer774 by co-evaporation of the Alq₃ and the coumarin 6. The Alq₃-coumarin6 weight ratio was adjusted to satisfy 1:0.003 so that the coumarin 6was dispersed in the Alq₃. The thickness of the layer 775 was set to be40 nm.

A second layer 776 including Alq₃ and lithium (Li) was formed on thelayer 775 by co-evaporation of the Alq₃ and the lithium. TheAlq₃-lithium weight ratio was adjusted to satisfy 1:0.01 so that thelithium was dispersed in the Alq₃. The thickness of the second layer 776was set to be 30 nm.

Next, a first layer 777 including NPB and molybdenum oxide was formed onthe second layer 776 by co-evaporation of the NPB and the molybdenumoxide. The molar ratio between the NPB and the molybdenum oxide (i.e.,molybdenum oxide/NPB) was set to be 1.25. At this moment, with respectto the light emitting element 5, the thickness of the first layer 777was set to be 0 nm. That is, the first layer 777 was not formed in thelight emitting element 5. With respect to the light emitting element 6,the thickness of the first layer 777 was set to be 100 nm. With respectto the light emitting element 7, the thickness of the first layer 777was set to be 120 nm. With respect to the light emitting element 8, thethickness of the first layer 777 was set to be 140 nm. With respect tothe light emitting element 9, the thickness of the first layer 777 wasset to be 160 nm. With respect to the light emitting element 10, thethickness of the first layer 777 was set to be 180 nm. With respect tothe light emitting element 11, the thickness of the first layer 777 wasset to be 200 nm.

Subsequently, a first electrode 778 was formed on the first layer 777 byvacuum evaporation of aluminum. The thickness of the first electrode 778was set to be 100 nm.

When the current flows through each of the above-manufactured lightemitting elements by applying the voltage thereto such that a potentialof the second electrode 772 is higher than that of the first electrode778, holes generated in the first layer 777 are injected in the firstelectrode 778 while electrons generated in the second layer 776 areinjected in the layer 775. The holes are injected in the layer 773 fromthe second electrode 772. The holes injected from the second electrode772 and the electrons injected from the second layer 776 are recombinedin the layer 775, allowing the coumarin 6 to emit light. Accordingly,the layer 775 serves as a light emitting layer. Further, the layer 773serves as a hole injecting layer. The layer 774 serves as a holetransporting layer. In each light emitting element of the presentembodiment, the substances included in the layer 775 and a substancewith an electron transporting property included in the second layer 776are both Alq₃ and have equivalent electron affinity.

FIG. 19 shows the voltage-luminance characteristics of the lightemitting elements according to the present embodiment, FIG. 20 shows thevoltage-current characteristics thereof, and FIG. 21 shows theluminance-current efficiency characteristics thereof. In FIG. 19, ahorizontal axis represents the voltage (V) while a perpendicular axisrepresents the luminance (cd/m²). In FIG. 20, a horizontal axisrepresents the voltage (V) while a perpendicular axis represents thecurrent (mA). In FIG. 21, a horizontal axis represents the luminance(cd/m²) while a perpendicular axis represents the current efficiency(cd/A). In FIGS. 19, 20 and 21, curves marked by ● indicate thecharacteristics of the light emitting element 5, curves marked by ▴indicate the characteristics of the light emitting element 6, curvesmarked by Δ indicate the characteristics of the light emitting element7, curves marked by ▪ indicate the characteristics of the light emittingelement 8, curves marked by □ indicate the characteristics of the lightemitting element 9, curves marked by ⋄ indicate the characteristics ofthe light emitting element 10, and curves marked by ∘ indicate thecharacteristics of the light emitting element 11, respectively.

According to FIG. 20, it is known that there is almost no difference inthe amount of current that flows through the respective light emittingelements upon applying the given voltage to the light emitting elementseven when the thicknesses of the first layers 777 having a function ofgenerating holes are varied. Meanwhile, it is also known that the amountof luminance upon applying the given voltage to the respective lightemitting elements is varied greatly depending on the thicknesses of thefirst layers 777 according to FIG. 19.

FIG. 22 is a graph in which the current efficiency (cd/A) with respectto a distance (nm) between the layer 775 and the first electrode 778 isplotted (marked by ●). The curve in FIG. 22 is an approximated curveshowing the change in current efficiency. Further, the currentefficiency is obtained when the light emitting element emits light atthe luminance of 1,000 cd/m². In FIG. 22, a horizontal axis representsthe distance (nm) while a perpendicular axis represents the currentefficiency (cd/A). According to FIG. 22, it is known that the currentefficiency is changed depending on the distance between the layer 775and the first electrode 778 (i.e., a sum of the respective filmthicknesses of the layer 775, the second layer 776 and the first layer777), and the current efficiency is gradually increased when thedistance between the layer 775 and the first electrode 778 is more than200 nm. It is thought that this phenomenon is caused due to the effectof interference of light, wherein when an optical distance between alight emitting region and the first electrode (i.e.,reflectance×distance) is (2m−1)/4 times (i.e., 1/4, 3/4, 5/4 . . .times) of the light emission wavelength, the light extraction efficiencyis increased, whereas when the optical distance therebetween is m/2times (i.e., 1/2, 1, 3/2 . . . times) of the emission wavelength, thelight extraction efficiency is reduced. Consequently, in the presentembodiment, by setting the thickness of the first layer 777 to be morethan 160 nm, light generated in the light emitting layer can be emittedto the outside effectively while preventing the short-circuiting betweenthe electrodes. In addition, a light emitting element having slightincrease in the resistance value that is caused by increase in thicknesscan be obtained.

The results of measuring the change in emission spectrum depending on anangle of seeing a light emitting surface with respect to the lightemitting elements 5, 7 and 11 are shown in FIGS. 23A, 23B and 23C,respectively. In FIGS. 23A, 23B and 23C, a horizontal axis represents awavelength (nm) while a perpendicular axis represents the emissionintensity (at an given unit).

The emission spectrum is measured by changing an angle of seeing thelight emitting surface, i.e., an angle between a normal line to thelight emitting surface and an normal line to an measurement surface,every 10° C. in a range of 0 to 70 degrees.

FIG. 23A shows the results of measuring the change in emission spectrumof the light emitting element 5. FIG. 23B shows the results of measuringthe change in emission spectrum of the light emitting element 7. FIG.23C shows the results of measuring the change in emission spectrum ofthe light emitting element 11.

In FIG. 23B, the emission spectrum is changed depending on the angle ofseeing the light emitting surface, wherein when the angle is less than30 degrees, the emission spectrum with about 507 nm shows a maximalvalue of the emission intensity and when the angle is more than 40degrees, the emission spectrum with about 555 nm shows a maximal valueof the emission intensity. Accordingly, it is known that the shape ofthe emission spectrum of the light emitting element 7 is largely changeddepending on the change in angle so that there is a major change inemission spectrum depending on the angle of seeing the light emittingsurface. On the other hand, in FIGS. 23A and 23C, although the emissionintensity is reduced with increasing the angle of seeing the lightemitting surface, the wavelength showing a maximal value of emissionintensity is not changed. Accordingly, it is known that with respect tothe light emitting elements 5 and 11, there is almost no variation inthe shape of emission spectrum in accordance with the change in angle,resulting in slight variation in emission spectrum depending on theangle of seeing the light emitting surface.

Embodiment 4

One embodiment of the light emitting element according to the presentinvention will be described. Further, a light emitting element of thisembodiment is similar to that of Embodiment 2, except that the molarratio between the NPB and the molybdenum oxide included in the secondlayer is different from that of the light emitting element of Embodiment2. Therefore, the light emitting element of this embodiment will bedescribed with reference to FIG. 14.

Indium tin oxide including silicon was formed over a substrate 731 bysputtering to form a second electrode 732 with a thickness of 110 nm. Asubstrate made of glass was used as the substrate 731.

Next, a layer 733 including molybdenum oxide and NPB was formed on thesecond electrode 732 by co-evaporation of the molybdenum oxide and theNPB. The thickness of the layer 733 was set to be 50 nm. The molar ratiobetween the molybdenum oxide and the NPB (i.e., molybdenum oxide/NPB)was adjusted to be 1.0.

A layer 734 including NPB was next formed on the layer 733 by vacuumevaporation of the NPB. The thickness of the layer 734 was set to be 10nm.

A layer 735 including tris(8-quinolinolato)aluminum (abbreviation: Alq₃)and coumarin 6 was formed on the layer 734 by co-evaporation of the Alq₃and the coumarin 6. The Alq₃-coumarin 6 weight ratio (i.e., Alq₃:coumarin 6) was adjusted to be 1:0.01 so that the coumarin 6 wasdispersed in the Alq₃. The thickness of the layer 735 was set to be 40nm. Further, the co-evaporation is an evaporation method that isperformed simultaneously from plural evaporation sources.

Next, Alq₃ was formed on the layer 735 by vacuum evaporation to form alayer 736 including the Alq₃ with a thickness of 10 nm.

A second layer 737 including Alq₃ and lithium (Li) was formed on thelayer 736 by co-evaporation of the Alq₃ and the lithium. TheAlq₃-lithium weight ratio (i.e., Alq₃:lithium) was adjusted to be 1:0.01so that the lithium is dispersed in the Alq₃. The thickness of thesecond layer 737 was set to be 10 nm.

A first layer 738 including NPB and molybdenum oxide was formed on thesecond layer 737 by co-evaporation of the NPB and the molybdenum oxide.The molar ratio between the NPB and the molybdenum oxide (i.e.,molybdenum oxide/NPB) was adjusted to be 2.0. The thickness of the firstlayer 738 was set to be 20 nm.

A first electrode 739 was formed on the first layer 738 by vacuumevaporation of aluminum. The thickness of the first electrode 739 wasset to be 100 nm.

When the current flows through the light emitting element manufacturedabove by applying the voltage thereto such that a potential of thesecond electrode 732 is higher than that of the first electrode 739,holes generated in the first layer 738 are injected in the firstelectrode 739 while electrons generated in the second layer 737 areinjected in the layer 736. The holes are injected in the first layer 733from the second electrode 732. The holes injected from the secondelectrode 732 and the electrons injected from the second layer 737 arerecombined in the layer 735, allowing the coumarin 6 to emit light.Accordingly, the layer 735 serves as a light emitting layer. Further,the layer 733 serves as a hole injecting layer. The layer 734 serves asa hole transporting layer. The layer 736 serves as an electrontransporting layer.

The voltage-luminance characteristics of the light emitting elementmanufactured according to the present embodiment are shown in FIG. 25.In FIG. 25, a horizontal axis represents the voltage (V) while aperpendicular axis represents the luminance (cd/m²). According to FIG.25, it is known that the light emitting element of the presentembodiment is operated favorably.

Embodiment 5

Another embodiment of the light emitting element according to thepresent invention will be described. A light emitting element of thepresent embodiment is similar to that of Embodiment 2, except that thesecond layer includes DNTPD rather than the NPB and the molar ratio ofsubstances included in the second layer is different from those of thelight emitting element of Embodiment 2. Therefore, the light emittingelement of the present embodiment will be described with reference toFIG. 14.

Indium tin oxide including silicon was formed on a substrate 731 bysputtering to form a second electrode 732 with a thickness of 110 nm.Further, a substrate made of glass was used as the substrate 731.

Next, a layer 733 including molybdenum oxide and NPB was formed on thesecond electrode 732 by co-evaporation of the molybdenum oxide and theNPB. The thickness of the layer 733 was set to be 50 nm. The molar ratiobetween the NPB and the molybdenum oxide (molybdenum oxide/NPB) wasadjusted to be 1.0.

A layer 734 including NPB was formed on the layer 733 by vacuumevaporation of the NPB. The thickness of the layer 734 was set to be 10nm.

A layer 735 including tris(8-quinolinolato)aluminum (abbreviation: Alq₃)and coumarin 6 was formed on the layer 734 by co-evaporation of the Alq₃and the coumarin 6. The Alq₃-coumarin 6 weight ratio (i.e., Alq₃:coumarin 6) was adjusted to be 1:0.01 so that the coumarin 6 wasdispersed in the Alq₃. The thickness of the layer 735 was set to be 40nm. Further, the co-evaporation is an evaporation method that isperformed simultaneously from plural evaporation sources.

Next, a layer 736 including Alq₃ was formed on the layer 735 by vacuumevaporation of the Alq₃. The thickness of the layer 736 was set to be 10nm.

A second layer 737 including Alq₃ and lithium (Li) was formed on thelayer 736 by co-evaporation of the Alq₃ and the lithium. TheAlq₃-lithium weight ratio (i.e., Alq₃:lithium) was adjusted to be 1:0.01so that the lithium was dispersed in the Alq₃. The thickness of thesecond layer 737 was set to be 10 nm.

Next, a first layer 738 including DNTPD and molybdenum oxide was formedon the second layer 737 by co-evaporation of the DNTPD and themolybdenum oxide. The molar ratio between the DNTPD and the molybdenumoxide (molybdenum oxide/DNTPD) was adjusted to be 3.1. The thickness ofthe first layer 738 was set to be 20 nm.

A first electrode 739 with a thickness of 100 nm was formed on the firstlayer 738 by vacuum evaporation of aluminum.

When the current flows through the above-manufactured light emittingelement by applying the voltage thereto such that a potential of thesecond electrode 732 is higher than that of the first electrode 739,holes generated in the first layer 738 are injected in the firstelectrode 739 while electrons generated in the second layer 737 areinjected in the layer 736. The holes are injected in the layer 733 fromthe second electrode 732. The holes injected from the second electrode732 and the electrons injected from the second layer 737 are recombinedin the layer 735 so that the coumarin 6 emits light. Accordingly, thelayer 735 serves as a light emitting layer. Further, the layer 733serves as a hole injecting layer. The layer 734 serves as a holetransporting layer. The layer 736 serves as an electron transportinglayer.

The voltage-luminance characteristics of the light emitting elementmanufactured according to the present embodiment are shown in FIG. 26.In FIG. 26, a horizontal axis represents the voltage (V) while aperpendicular axis represents the luminance (cd/m²). According to FIG.26, the light emitting element having the structure of the presentembodiment is operated favorably.

What is claimed is:
 1. A light-emitting device comprising: a transistorover a substrate; a first electrode over and electrically connected tothe transistor; a first layer over and in direct contact with the firstelectrode, the first layer being configured to generate a hole; a secondlayer over and in direct contact with the first layer, the second layerbeing configured to generate an electron; a third layer over the secondlayer, the third layer being configured to emit light; and a secondelectrode over the third layer, wherein the first layer is a singlelayer, wherein the second layer is a single layer, wherein an electrontransporting property of the second layer is stronger than a holetransporting property of the second layer, and wherein the firstelectrode and the second electrode are a cathode and an anode,respectively.
 2. The light-emitting device according to claim 1,wherein: the first layer comprises a first substance having a holetransporting property and a second substance having an electronaccepting property to the first substance; and the second layercomprises a third substance having the electron transporting propertyand a fourth substance having an electron donating property to the thirdsubstance.
 3. The light-emitting device according to claim 2, wherein:the hole transporting property of the first substance is stronger thanan electron transporting property thereof; and the electron transportingproperty of the third substance is stronger than a hole transportingproperty thereof.
 4. The light-emitting device according to claim 1,wherein: the first layer comprises a p-type semiconductor; and thesecond layer comprises an n-type semiconductor.
 5. The light-emittingdevice according to claim 4, wherein: the first layer further comprisesan electron-accepting substance with respect to the p-typesemiconductor; and the second layer further comprises anelectron-donating substance with respect to the n-type semiconductor. 6.The light-emitting device according to claim 1, wherein the second layeris in direct contact with the third layer.
 7. The light-emitting deviceaccording to claim 1, wherein the second electrode is configured totransmit light emitted from the third layer.
 8. A light-emitting devicecomprising: a substrate; a pixel portion over the substrate; a wiringover the substrate, the wiring electrically connected to the pixelportion; and an external input terminal electrically connected to thewiring, wherein: the pixel portion comprises a plurality of pixelsarranged in a matrix form; and at least one of the pixels comprises: atransistor; a first electrode over and electrically connected to thetransistor; a first layer over and in direct contact with the firstelectrode, the first layer being configured to generate a hole; a secondlayer over and in direct contact with the first layer, the second layerbeing configured to generate an electron; a third layer over the secondlayer, the third layer being configured to emit light; and a secondelectrode over the third layer, wherein the first layer is a singlelayer, wherein the second layer is a single layer, wherein an electrontransporting property of the second layer is stronger than a holetransporting property of the second layer; and wherein the firstelectrode and the second electrode are a cathode and an anode,respectively.
 9. The light-emitting device according to claim 8, furthercomprising a driver circuit electrically connected to the external inputterminal through the wiring.
 10. The light-emitting device according toclaim 9, wherein the driver circuit is provided over the substrate. 11.The light-emitting device according to claim 9, wherein the drivercircuit is provided outside the substrate.
 12. The light-emitting deviceaccording to claim 8, further comprising an IC chip over the externalinput terminal.
 13. The light-emitting device according to claim 8,further comprising a printed wiring board connected to the externalinput terminal.
 14. The light-emitting device according to claim 8,wherein: the first layer comprises a first substance having a holetransporting property and a second substance having an electronaccepting property to the first substance; and the second layercomprises a third substance having the electron transporting propertyand a fourth substance having an electron donating property to the thirdsubstance.
 15. The light-emitting device according to claim 14, wherein:the hole transporting property of the first substance is stronger thanan electron transporting property thereof; and the electron transportingproperty of the third substance is stronger than a hole transportingproperty thereof.
 16. The light-emitting device according to claim 8,wherein: the first layer comprises a p-type semiconductor; and thesecond layer comprises an n-type semiconductor.
 17. The light-emittingdevice according to claim 16, wherein: the first layer further comprisesan electron-accepting substance with respect to the p-typesemiconductor; and the second layer further comprises anelectron-donating substance with respect to the n-type semiconductor.18. The light-emitting device according to claim 8, wherein the secondelectrode is configured to transmit light emitted from the third layer.19. An electronic appliance comprising the light-emitting deviceaccording to claim 8.